toolchain/toolchain-external/toolchain-external-codesourcery-amd64: remove package

toolchain/toolchain-external/toolchain-external-codesourcery-amd64: remove package

This toolchain uses an old gcc 6.2.0 compiler (not even the latest gcc
from the 6.x series), which fails to build the recent Boost
package. Since newer versions of this toolchain are no longer made
publicly available from Mentor Graphics, our only option is to drop
the toolchain.

Fixes:

  http://autobuild.buildroot.net/results/10edaed22c15b9d0f7de187085aeebc96e5ebe6c/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>